Tube Screamer Clone Mod

[duhvoodooman***]

For you hardware geeks like myself, I thought I'd provide some additional details on the clipping diode setups. To me, this is the most important single feature within an overdrive pedal, because its primary function is to "clip" the input signal, thus distorting its sound. The debates about using the "true TS-808 chip" in a Screamer clone seem pretty funny to me, since the difference between a NOS JRC4558D, current production JRC4558D, or an RC4558P opamp are very subtle, compared to what you get by changing diode types.

As I understand it, diodes are essentially a one-way electrical gate, allowing current to pass through them in only one direction. Diodes apparently require a threshold voltage to start working, and different types "turn on" faster than others. So when the input signal exceeds the threshold voltage, the diode turns on and restricts the current flow, i.e. "clips" the signal. The turn-on time determines how "hard" or "soft" the clipping is, with slower turn-on giving a softer clip.

For these clipping applications, diodes are typically used in pairs, wired together in parallel but opposite current flow orientations. The opposite polarities allow one diode to clip the positive signal (i.e. top half of the sine wave) while the other clips the negative signal (bottom half). Use of such a matched pair of diodes is referred to as "symmetrical" clipping, since both signal polarities are clipped the same way. However, you can use two different types of diodes or place additional diodes in series on one side vs. the other to give "asymmetrical" clipping, where the two signal polarities are clipped differently. This is reputed to give a more natural or tube-like tone. Such arrangements are very popular in modified TS-type pedals offered by many of the "boutique" pedal makers.

The original TS-808 used two 1N914 silicon diodes for a symmetrical clip. Silicon diodes have a fairly high threshold voltage, and turn on quickly, i.e. "hard". However, I opted to use an asymmetrical arrangement of one 1N914 and two 1N4001 diodes for a somewhat smoother character. This was included as an option in the BYOC Overdrive pedal kit I started with on this project. So, my "stock" TS-808 tone isn't 100% absolutely stock, but it's still very close. The difference is pretty subtle.

For the second clipping mode, I chose 1N34A germanium diodes in an asymmetrical 2-and-1 arrangement. Germanium diodes have a lower turn-on voltage than silicon, but also turn on more slowly, giving a softer clip. The resulting tone is significantly smoother than the stock TS-808, and it reminds me a lot of the overdrive tone I get from my Delta Blues tube amp when I push the clean channel volume way up to saturate the power tubes and run the output through an attenuator to knock the volume back down. I assume it relates to the lower voltage threshold, but running through the germanium clipping diodes really reduces the overall pedal output volume significantly, so you have to turn up the Level knob to maintain overall volume. But it's a really nice, restrained, bluesy overdrive!

The last diode combo I used was two 3mm yellow LED's in a straight symmetrical arrangement. Use of LED's for signal clipping is at the heart of the very expensive ($379!!) Landgraff Dynamic Overdrive pedal, which is little more than a TS-808 with switchable silicon diodes and LED's for the clipping section. People rave about this pedal and some other similar LED-based overdrives because of their aggressive "Marshall Plexi" sound. LED's have a higher voltage threshold than silicon, and they turn on "hard". They cut the signal least overall, so produce the highest (loudest) output of these three combos. I have to admit, I like the sound of the LED clipping mode, especially with humbuckers.

[M29]

Hello duhvoodooman,

Man this stuff is well appreciated! I am curious what relationship our guitar picks have to the level it takes to break the threshold. Is it attack that breaks or the volume? Say the attack of our picks or the volume that the amp is turned up to? I guess what I am curious about is, does it make much difference in volume to break the threshold of the diodes and get this thing singing and would either one work better at low volume compared to a louder volume. Or maybe it just has to do with the smoothness of the breakup itself.
I understand how it works but maybe not compared to our playing.
As I read my own post I don't know if I am saying exactly what I am trying to say. Sorry...

Thanks duhvoodooman for all your time and help on this I am getting a lot out of this. My only problem is I just recently got a Boss DS-1. Now I wants a clone like yours

M29

[duhvoodooman***]

I got to crank up the volume a little higher tonight while I had the house to myself. Here are two more clips using the Epi LP through the modded pedal. The first is basically a repeat of the third clip in my previous posting--the Who's Won't Get Fooled Again outro riff, but with more authority this time. Repeated 4 times--first with no pedal, then germanium diodes (soft clip), then silicon diodes (regular TS type), and lastly with the hard-clipping yellow LED's. The latter is the only way to go for this tune, IMO!

The second is just a little of the familiar ZZ Top Tush riff, using the yellow LED setting with the drive at about 3/4. The gain range is stock TS-808, as is the bass rolloff frequency.

WGFA

Tush

[Spudman***]

Meet the new boss - same as the old boss Yeaaaaaaaaaaaaaaaahhhhhhhhhh!

That sounded really good. Very nice piece of circuitry you got going on. Congratulations too. You've got me gassing again. You won't get gassed again. Yeaaaaaaaaaaaahhhhhhhhhhh!

[duhvoodooman***]

I am curious what relationship our guitar picks have to the level it takes to break the threshold. Is it attack that breaks or the volume? Say the attack of our picks or the volume that the amp is turned up to? I guess what I am curious about is, does it make much difference in volume to break the threshold of the diodes and get this thing singing and would either one work better at low volume compared to a louder volume. Or maybe it just has to do with the smoothness of the breakup itself.
I understand how it works but maybe not compared to our playing.
As I read my own post I don't know if I am saying exactly what I am trying to say. Sorry....

I think I understand your point, M29. As I understand it, it's the strength of the input signal that the pedal sees that determines how much the diodes clip at a given gain setting, and that can be modulated either through pick attack or the guitar's volume setting. So picking softly or rolling back the volume knob will definitely clean up the sound. Now, once the clipped signal leaves the pedal, there's obviously a lot more that can happen to it through the remaining pedal-preamp-power stage-speaker chain. I would assume that if you had a perfectly clean chain--no sources of harmonic distortion at all, no matter the volume setting--then you'd just hear the pedal's output amplified. But that pretty much never happens, even with a "clean" amp like a big ol' Fender Twin. There's always going to be some degree of further "character" added to the sound, and it will be volume-dependent. For amps that break up substantially with increased volume--or with amps where additional distortion is intentionally added--the pedal's output signal will be altered very substantially. What finally comes out of the amp speaker may sound great or not-so-great, depending upon how that interaction occurs.